A “day” begins in any particular locality when the sun rises in that
place; but officially a day begins at precisely twelve o’clock,
midnight, and lasts until midnight, twenty-four hours later, when the
next day begins. But this is a shifting point, constantly moving as the
earth revolves on its axis, and it will be seen that this would be very
unsatisfactory for official science, which would thus have no “check,”
so to speak, as to where any particular day actually begins. In order
to establish this, what is known as an Official Date Line has been
fixed, which is the International Standard for the beginning of each
new day.
The meridian running through Greenwich (London) is accepted as the
standard meridian; meridians east and west of this are drawn upon the
maps, and marked, so many degrees east or west, as the case may be.
The 180th meridian (half way round the globe) fortunately falls about
the middle of the Pacific Ocean, where there is no land, and this has
been settled upon as the international date line. This line is slightly
bent, in two places--in the southern hemisphere, to include certain
small islands close to New Zealand (so that they may have the same
time) and, in the northern hemisphere, it is bent first to the left and
then to the right--the former to include certain small islands adjacent
to Alaska, and the latter to permit the inclusion of all the main land
of Siberia. This may be seen by referring to any map or globe of the
world.
While a day at any particular place is twenty-four hours long, each
day lasts on earth at least forty-eight hours. Any given day, say
Christmas, is first counted as that day just west of the date line. An
hour later, Christmas begins 15 degrees west of that line, two hours
later it begins 30 degrees west of it, and so on round the globe. The
people just west of the date line, who first hailed Christmas, have
enjoyed twelve hours of it when it first reaches England, eighteen
hours of it when it begins in Central United States, and twenty-four
hours of it, or a whole day, when it begins in Western Alaska, just
east of the date line. Christmas, then, has existed twenty-four hours
on the globe, but, having just begun in Western Alaska, it will tarry
twenty-four hours longer among mankind, making forty-eight hours that
the day blesses the earth.
If the date line followed the meridian 180 degree without any
variation, the total duration of a day would be exactly forty-eight
hours as just explained. But this line is quite irregular, as shown
on the map. Because of this irregularity of the date line, the same
day lasts somewhere on earth about forty-nine hours! Suppose we start
at Cape Deshnef, Siberia, longitude 169 degrees West, a moment after
midnight on the 3d of July. The 4th of July has begun, and, as
midnight sweeps round westward, successive places see the beginning of
this day. When it is the 4th in London, it has been the 4th at Cape
Deshnef twelve hours and forty-four minutes. When the Glorious Day
arrives in New York, it has been seventeen hours and forty-four minutes
since it began at Cape Deshnef. When it reaches our most western
point on this continent, Attu Island, 173 degrees East, it has been
twenty-five hours and twelve minutes since it began at Cape Deshnef.
Since it will tarry twenty-four hours at Attu Island, _forty-nine hours
and twelve minutes_ will have elapsed since the beginning of the day,
until the moment when all places on earth cease to count it that day.
Portions of three days exist at the same time between 11:30 o’clock,
a.m., and 12:30 p.m., London time. When it is Monday noon at London,
Tuesday has begun at Cape Deshnef, but Monday morning has not yet
dawned at Attu Island; nearly half an hour of Sunday still remains
there.